The present invention provides a novel, optionally surfactant-free process for controlling particle number, particle size, and/or particle size distribution. The method of the invention also significantly improves monomer to polymer conversion rates, as well as, increases the level of polymer solids which can be emulsified/dispersed in water without polymer flocculation. Specifically, the improved process provides an aqueous dispersion of up to about 50% polymer solids in water in the absence of surfactant. The resulting fluoropolymer dispersions may be used in resin and coating applications.
Methods for preparing solid homopolymers of chlorotrifluoroethylene (hereinafter sometimes referred to as xe2x80x9cCTFExe2x80x9d, and which polymers are sometime referred to as xe2x80x9cPCTFExe2x80x9d) as well as copolymers thereof including but not limited to CTFE-vinylidene fluoride, CTFE-tetrafluoroethylene, and CTFE-ethylene copolymers are well-known in the art. See for example, Encyclopedia of Polymer Science and Engineering (John Wiley and Sons, 2nd Ed., vol.3, pg 463,1985); and U.S. Pat. Nos. 2,705,706; 2,700,622; 2,689,241; 2,569,524; 2,783,219; 2,820,026; 3,640,985; 3,671,510; 3,642,754; 3,632,847; and 3,014,015. As therein described, articles and films formed from PCTFE materials exhibit desirable vapor barrier properties, good thermal stability and resistance to strong oxidizing agents.
More specifically, processes suitable for the formation of the high molecular weight PCTFE homopolymers include, for example, free radical initiated polymerization either as bulk, suspension, or aqueous emulsion via the use of either a suitable initiator system, or alternatively with ionizing radiation. Copolymers of PCTFE, such as copolymerized PCTFE with vinylidene fluoride (xe2x80x9cVDFxe2x80x9d) and/or tetrafluoroethylene (xe2x80x9cTFExe2x80x9d) are commonly produced via either suspension or emulsion polymerization processes. Generally, however, the use of comonomers is such that the weight percentage of the comonomers is relatively low, i.e., generally comprising a minor proportion of the total polymer as it is known that the desirable vapor barrier properties are substantially degraded by the inclusion of excessive amounts of comonomers.
While these processes provide useful methods for the production of PCTFE polymer resins and dispersions, various shortcomings in one or more of these processes have compelled the development of alternative novel methods for the production of PCTFE aqueous dispersions and polymer resins, a process which could be readily applied to other fluoroolefins.
One such shortcoming in the prior art is the use of high levels of expensive fluorinated surfactants in the conventional emulsion polymerization of fluoroolefins and the use of moderate quantities of fluorinated surfactants for suspension polymerizations (granular or slurry) which are difficult to recover. If the intention is to produce resin, the presence of even a small amount of such surfactants or soaps produces deleterious effects, i.e. promotes and stabilizes foam, impairs intercoat adhesion, and causes agglomeration of ordinarily immiscible media used for their separation. A method for recovering these fluorinated surfactants from spent polymerization mixtures is disclosed in U.S. Pat. No. 4,623,487. In coatings applications, the use of high levels of nonionic or ionic surfactants leads to discoloration upon high temperature processing, or in the case of room temperature coalescable coatings, leads to films with lower moisture and chemical barrier properties.
The present invention is a process improvement over the disclosures made in U.S. Pat. No. 5,453,477. In U.S. Pat. No. 5,453,477 a method is disclosed for the production of PCTFE/VDF resin suspensions using a catalyst system comprising t-butylhydroperoxide, sodium-m-bisulfite, and iron (II) sulfate hydrate. Although the disclosures in U.S. Pat. No. 5,453,477 describe stable resin suspensions, the process has been significantly improved. In this invention, a surfactant free emulsion polymerization method is disclosed which describes the production of highly stable PCTFE/VDF emulsions which can be obtained in: (1) higher percentages of stable self-emulsifiable fluoropolymer solids in water (42-48%) (2) at faster conversion rates of monomer to polymer and (3) in smaller particle sizes (0.17 microns). This is obtained by carefully tailoring the particle nucleation step and particle number generated in the early phase of polymerization. The fluoropolymer is best described as a self-emulsifying fluoropolymer mass whose polymer chains have sufficient quantities of terminal sulfonic acid end groups, such that fluoropolymer emulsions can be prepared having 40-48% percent solids in water, in the absence of added surfactant.
Although U.S. application Ser. No. 08/207,973 filed on Mar. 7, 1994 (pending) and U.S. application Ser. No. 08/386,787 filed on Feb. 10, 1995 (pending) have addressed novel methods for polymerizing fluoroolefins in the absence of surfactants, these applications do not relate to how the particle size, particle number, and particle distribution, and thus the latex stability of the resulting fluorolefins may be controlled. It has been found that this inability to control the latex stability of the fluoroolefins often results in product coagulation during polymerization and thus a reduction in product yields.
Therefore, there remains a present and continuing need for the provision of fluoropolymer aqueous dispersions free from undesirable stabilizing additives such as ionic or nonionic surfactants, defoamers, and hydrocarbon perfluorinated or partially perfluorinated polar functional monomers required by the prior art for imparting stability to aqueous dispersions. There also remains a continuing need in the art for the production of improved fluoropolymers such as PCTFE homopolymers and copolymers thereof which feature improved machine processability, particularly in regard to conventional thermoforming and process equipment, and improved concentration or purification of the aqueous dispersions formed therefrom. It would further be desirable to provide a process for controlling the particle size, number, and/or distribution of fluoropolymers or copolymers thereof in aqueous dispersions in order to increase product yields and reduce coagulation.
In one embodiment, the invention relates to a process for controlling particle number and/or particle size of a fluoropolymer in an aqueous dispersion, in the absence of surfactants, comprising:
(a) reacting a mixture comprising a fluoroolefin, water and a radical initiator system comprising a reducing agent and an oxidizing agent wherein said radical initiator system is added in at least two charges, an initial charge and a continuous charge, and wherein the amount of said reducing agent in the initial charge is from about 100 to about 10,000 ppm based upon the total weight of polymer produced and the amount of oxidizing agent in the initial charge is from about 100 to about 10,000 ppm based upon the total weight of polymer produced.
This process produces an aqueous fluoropolymer dispersion having a particle size between about 0.01 microns and about 1.0 micron and a polymer solids content of about 50 weight percent all at conversion rates that are much higher than those obtained using processes of the prior art. See Example 10 and Comparative Example 2.
The invention also relates to stable, aqueous dispersions of fluoropolymer which may be used in resin and coating applications.